Flexible electrical jumpers such as gun shunts used in spot welding machines may be made from tempered copper sheets or laminates such as half hard sheets. The tempered characteristic may be described as somewhat dead and they have little spring tendency. Current production techniques for such shunts utilize rivets to hold the laminates together. This results in a high initial resistance, and one which increases with time due primarily to oxidation of the copper laminates. The higher resistance reduces operating efficiency by increasing operating costs and also increases the shunt operating temperatures which reduces shunt life also accelerating oxidation. This change in resistance over the life of the shunt adversely changes the quality of the weld. It is accordingly desirable to have a flexible jumper such as a gun shunt which has the lowest possible resistance levels and lower power losses, which in turn produce more efficiency in the spot welding process resulting in lower operating costs. It is also important that the shunt have cooler shunt operating temperatures which result in longer shunt life and in consistent quality welds.
Press welding has been employed in the fabrication of cables and bus bars or arms, examples being seen in prior U.S. Pat. Nos. 4,455,659 and 4,640,982. The press welding process utilizes graphite electrodes which are movable with respect to each other to clamp the work at substantial pressure. The welding current passing from one electrode to the other through the clamped work creates a very high heat which approaches the fusion temperature of the metal of the clamped work. In fact, partial fusion does take place under the carefully controlled temperature and pressure of the process.
Excessive temperature or pressure can damage or destroy the work. Proper pressure welding of laminates for example can produce electrical resistance or loss characteristics essentially the same as in a solid conductor of the same material.
However, pressure welding creates so much heat that any temper in the metal of the laminates is altered or destroyed, which is no doubt one of the reasons that conventional jumpers such as gun shunts are formed with rivets. Accordingly, it would be desirable to make jumpers such as gun shunts with pressure welding techniques. However, to do so would require a pressure welding process where the balance of the item being welded is essentially isolated from the heat of the welding process.